How do I orientate an object exactly flat to a plane?
This is where it would be handy to have Maya's Bake Pivot Orientation command.  Sadly, I haven't been able to find a straightforward way to do this.  The best method I have come up with so far is as follows:


Set the mesh object's Origin to Geometry.
Create a custom Transform Orientation for the face you want to be considered 'up'

Create an Empty at the object's origin
With the Empty selected, use Object, Transform, Align to Transform Orientation
Parent the mesh object to the Empty

With the Empty selected, use Object, Clear, Rotation.  The mesh object's orientation will now be reset to 'normal'
Optionally, you can apply the rotation transform on the mesh object.